The process and apparatus of present invention concern the thermal cycles of molding expandable polystyrene (hereinafter "EPS") into useful articles made of what is commonly called "styrofoam".
Known methods for molding EPS begin with the step of filling a mold with EPS beads. The beads are small hollow polystyrene spheres filled with a gas-forming expansion agent, typically pentane. The molds are usually made of cast aluminum and consist of two halves. Each half is mounted onto a platen to form a "mold assembly" and create a steam cavity behind each side of the mold. Generally, one or both of the platens is moveable to allow separation of the two sides of the mold and thereby facilitate removal of the item being produced.
Once the mold is filled with beads, the next step in EPS molding is a heat cycle to heat the mold and beads to expand and fuse the beads to form the product. The steam cavity behind one half of the mold is filled with steam. This steam is drawn through core vents into the bead-filled mold cavity.
As shown below, the energy required to melt and mold the EPS beads is insignificant compared to the energy used to heat the mold and surrounding steam chest (i.e., the mold/chest).
The heat required to melt and mold a one pound shot of EPS beads is 86 ETUs. The melt temperature of EPS beads is 230.degree. F. and the fusion temperature is 240.degree. F. The specific heat of EPS is 0.24 BTUs/lb-.degree.F. and the latent heat of fusion 50 BTUs/lb. The energy required to melt and mold EPS is given by the following formula. EQU Energy=Specific Heat.times..DELTA.Temperature+Latent Heat of Fusion(A)
Therefore, the energy required to melt and fuse EPS is at an ambient temperature of 80.degree. F. is 0.24 BTUs/lb-.degree.F..times.(230.degree. F.-80.degree. F.)+50 BTUs/lb=86 BTUs/lb.
After the EPS beads were melted and fused, the temperature of the mold and surrounding steam chest had to be reduced from 240.degree. F. to 210.degree. F. to allow for the molded EPS article to be removed from the mold. The heat required to raise the temperature of the mold and surrounding steam chest from the demold temperature of 210.degree. F. to the EPS fusion temperature of 240.degree. F. is on the order of 5,190 BTUs. The mold is typically formed from aluminum which has a specific heat of 0.21 BTU/lb-.degree.F. The mold will be assumed to have a mass of 300 lbs. The energy required to raise the temperature of this mold is given by the following formula. EQU Energy=Mass of Material.times.Specific Heat.times..DELTA.Temperature(B)
Therefore, the energy required to raise the temperature of this aluminum mold from 210.degree. F. to 240.degree. F. is 300 lbs.times.0.21 BTU/lb-.degree.F..times.(240.degree. F.-210.degree. F.)=1,890 BTUs. The surrounding steam chest is typically formed from iron which has a specific heat of 0.11 BTU/lb-.degree.F. The surrounding steam chest will be assumed to have a mass of 1,000 lbs. Employing formula (B) above, the energy required to raise the temperature of this surrounding steam chest from 210.degree. F. to 240.degree. F. to 1,000 lbs..times.0.11 BTU/lb-.degree.F..times.(240.degree. F.-210.degree. F.)=3,300 BTUs.
Thus, for each one pound shot of EPS molded by the prior art batch process wherein the mold and the EPS beads were both brought to a temperature of 240.degree. F., the energy required is 5,276 BTUs (i.e., 86+1,890+3,300) per batch.
U.S. Pat. Nos. 4,801,361 and 4,813,859 to Calvin P. Bullard and F. Paul Szubelick disclose an improved method and apparatus for molding EPS. These patents disclose that reductions in energy requirements and length of the heating and cooling cycle can be achieved by concentrating the heat directly on the EPS beads alone, rather than on the combination of the surrounding steam chest, the mold and the EPS beads.
The inventor, Calvin P. Bullard, has continued to perfect and improve the method and apparatus for molding EPS that was jointly conceived by Calvin P. Bullard and F. Paul Szubelick (now deceased) and disclosed in U.S. Pat. Nos. 4,801,361 and 4,813,859. It has been observed by the inventor that the maximum potential reduction in energy requirements provided by the Bullard-Szubelick method and apparatus was not being realized as a result of ambient air and residual moisture remaining in the mold cavity from the previous cooling cycle.
Specifically, during testing conducted in connection with his U.S. Pat. Nos. 4,801,361 and 4,813,859, the inventor, Calvin P. Bullard, observed that while the steam temperature in a 3-inch steam supply pipe at a distance of three (3) feet from the mold/chest was 300.degree.-312.degree. F., the steam temperature within the mold/chest was only 210.degree.-215.degree. F. This wasteful reduction in steam temperature was determined to be attributable to two factors. The first being an unintended loss of superheat, or the desuperheat of the steam supply. As the steam flows from the 3-inch supply pipe into the mold/chest, there is a sudden enlargement of area with an attendant drop in the pressure and temperature of the steam. The reduction in temperature is caused by air and residual moisture in the mold/chest. That is, energy is used to heat the ambient air and residual moisture as well as the EPS beads. In this environment, desuperheating of the steam occurs with an immediate loss of energy. This is believed to be an industry-wide problem. The second factor being inadequate velocity/volume of the steam supplied to the mold/chest due to a poorly designed steampipe system.
It has also been observed that molded EPS articles tend to stick to the mold. This sticking of the molded EPS article to the mold is attributable, in part, to "post-expansion" which is the incomplete fusion of EPS beads and the incomplete extraction of the gas expansion agent from the EPS beads which inhibit proper curing of the EPS article prior to removal from the mold.
It is therefore an object of the present invention to provide a novel method and apparatus for molding expandable polystyrene, which is both energy and time efficient, comprising the steps of isolating the mold/chest and applying a vacuum purge to remove air and residual moisture from the mold/chest.
It is a further object of the present invention to provide a novel method and apparatus for molding expandable polystyrene which reduces the sticking of the molded article to the mold.
Additional objects and advantages of the invention will be set forth, in part, in the description which follows and in part will be apparent from this description, or may be learned by the practice of the invention. The objects and advantages of the invention are realized and obtained by means of the methods, apparatus and the combinations particularly pointed out in the appended claims.